This invention pertains to color projection apparatus. More specifically, this invention pertains to color projection apparatus operable to separate an illumination-light flux into multiple colors of light, create color images, combine such color images, and project the combined color images onto a viewing surface.
A prior-art color projection device using reflection-type light valves, shown in FIG. 5, is disclosed in Japanese Unexamined Patent Publication No. 63-39294. In the FIG. 5 apparatus, a white illumination-light flux is emitted from a light source 223 that comprises, for example, a halogen lamp. The illumination-light flux typically passes through a collimating lens 222 operable to make parallel the rays comprising the illumination-light flux. The illumination-light flux then enters a polarizing beam splitter (PBS) 221 disposes along the optical axis O of a color separation optical system 211.
S-polarized light of the illumination-light flux is reflected by the PBS 221 and is incident on the color-separation optical system 211. The s-polarized light incident on the color-separation optical system 211 is separated into the three primary colors, red (R), blue (B), and green (G), as follows.
The color separation optical system 211 includes a first prism 211A, a second prism 211B, and a third prism 211C, each disposes as shown in FIG. 5. A surface 211e of the first prism 211A is coated with an evaporated, thin dichroic film which reflects blue light but transmits light with longer wavelengths (i.e., red and green light). There is a gap between the first prism 211A and the second prism 211B. A thin, dichroic film, which reflects red light but transmits green light, is coated on a surface 211f of the second prism 211B, between the second prism 211B and the third prism 211C.
As the illumination-light flux reflected from the PBS 221 enters through surface 211a of the first prism 211A, blue light is reflected by the surface 211e and is then reflected inwardly by the surface 211a toward an emergence surface 211b of the first prism. Red light that passes through the surface 211e of the first prism 211A is reflected by the surface 211f and is then reflected inwardly by the surface of the second prism 211B between the first and second prisms. The inwardly reflected red light then exits through an emergence surface 211c of the second prism 211B. Green light that passes through the surface 211e of the first prism 211A and through the surface 211f of the second prism 211B travels toward an emergence surface 211d of the third prism 211C.
Reference numerals 212, 213, and 214 denote two-dimensional reflection-type liquid crystal light valves (LCLVs) for displaying a blue light image, a red light image, and a green light image, respectively. Each of the reflective-type LCLVs have dielectric reflecting layers 215, 216, and 217, respectively, formed on the back of transmission-type LCLVs so that the LCLVs operate as reflection-type LCLVs. As each color of light enters a respective LCLV, the light is modulated by the respective LCLV. Hence, each color""s video signal is converted into an image that has a transmission-rate distribution at the respective LCLV.
The modulated color light is then reflected and changed in polarization state by 90xc2x0. That is, the s-polarized light is converted by the LCLV to p-polarized light. The modulated and reflected color lights travel along reverse paths through the first, second and third prisms 211A, 211B, 211C, respectively, to be combined into a single light flux. The resultant combined, single light flux emerges from the incidence plane of the first prism 211A. The light flux whose polarization state has been converted is passed through the PBS 221 and projected on a screen 235 by a projection lens 224.
A problem with the conventional example shown in FIG. 5 is its inability to provide sufficiently high-contrast projected images. The conventional projection device example described herein does not project an idea xe2x80x9cblackxe2x80x9d image on the screen for the following reasons.
As linearly polarized light fluxes are incident on the dichroic films, after being passed through the PBS 221, the light flux is in part transmitted and in part reflected by the dichroic films. The s-polarized light and p-polarized light of the transmitted and reflected light fluxes are determined by a vector n normal to each dichroic film surface and a propagation vector T of the incident light flux. The vectors s of the s-polarized light is determined by s=nxc3x97T. Whenever a light flux is incident on a dichroic film in a manner such that the linear polarization plane is not in a plane defined by a line normal to the dichroic film surface and the propagation direction of the incident light, the light flux is separated into s-polarized light and p-polarized light, and a phase difference is imposed between the s-polarized light and p-polarized light. As a result, the resultant light flux typically behaves as elliptically polarized light. Hence, the light flux traveling through the PBS 221 includes light of undesirable polarization. The PBS 221 then directs the undesirable polarized light toward the screen 225. Accordingly, an ideal black image is not projected on the screen 225. Additionally, the contrast of the projected image is poor.
One known technique to solve the above problem has been disclosed in Japanese Unexamined Patent Publication No. 6-175123 wherein a special compensation plate is utilized to compensate for the light-flux polarization phase difference. In this technique, however, it is required to precisely control the light flux state of polarization at both the dichroic films and the compensation plate.
Another prior-art color projection device, shown in FIG. 6, is disclosed in Japanese Examined Patent Publication No. 5-82793. In the FIG. 6 apparatus, a plurality of dichroic films are operable to separate a white illumination-light flux emitted from a light source into the three primary light colors of red (R), green (G), and blue (B). The separated color lights are directed to respective LCLVs that modulate the color lights. Each color light, including the modulated color light, is reflected or emitted according to signals associated with the color light. Dichroic films are used to combine the color light and the resulting combined color light is projected on a screen by a projection lens.
The prior art color projection device of FIG. 6 includes a white illumination-light source comprising a lamp 20 and an elliptical mirror 21. Light rays comprising the illumination-light flux emitted from the light source 20 are made substantially parallel by a collimator lens 22. The light flux then passes through an opening 23a, defined by a light-interceptive plate 23, and a filter 24 operable to allow only visible light to pass therethrough. The light flux is directed toward a polarizing beam splitter (PBS) 16. The light flux incident on the PBS 16 is separated into s-polarized light, which is reflected by the PBS 16, and p-polarized light which is transmitted by the PBS 16 and subsequently discarded.
The s-polarized light is separated into blue light, red light, and green light by a blue-light reflection dichroic prism 17 and a red-light reflection dichroic prism 18. The separated color lights are routed to respective LCLVs 1B, 1G, and 1R. The color lights are modulated by the LCLVs 1B, 1G, and 1R to p-polarized light. Hence, each color""s video signal is converted into an image that has a transmission-rate distribution at the respective LCLV. The modulated color lights are reflected from the respective LCLVs 1B, 1G, and 1R, back to the corresponding dichroic prisms 17 and 18.
The color lights are then combined by the dichroic prisms 17 and 18 and directed to the PBS 16. The PBS 16 transmits p-polarized light (i.e., signal light) and directs the combined p-polarized light flux toward a screen 26 through a projection lens 25. The dichroic prisms 17 and 18 comprise the prior art color separation/combination optical system.
In the above-described prior art color projection device, a considerable amount of light is wasted (i.e., does not contribute to the projected image) resulting in an image having poor color balance and poor contrast. Consequently, an ideal black image cannot be projected. The reason for the poor color balance, poor contrast, and poor black image appears to relate to the dichroism of the dichroic prisms employed in the prior art device.
More particularly, depending upon the polarization state of the light flux, the dichroism of a dichroic prism may vary significantly. FIG. 7 illustrates the variance in dichroism of a red light reflection dichroic prism similar to the dichroic prism utilized in the prior art color projection device shown in FIG. 6.
The curve of FIG. 7 is based on an assumption that the s-polarized light and the p-polarized light fall on the dichroic film of the dichroic prism at an angle of incidence of about 45xc2x0. In FIG. 7, transmittance values are recorded on the Y-axis and wavelength values on the X-axis.
As illustrated by the curve in FIG. 7, p-polarized light having wavelengths less than about 500 nm and serving as a p-polarized light boundary wavelength is substantially reflected. P-polarized light having wavelengths greater than 500 nm is substantially transmitted. By contrast, s-polarized light having wavelengths less than about 580 nm, serving as an s-polarized light boundary wavelength, is substantially reflected, while s-polarized light having wavelengths greater than 580 nm is substantially transmitted.
Accordingly, with a dichroic prism, there exits a substantial difference between the transmission/reflection boundary wavelength for p-polarized light (about 500 nm) and the transmission/reflection boundary wavelength for s-polarized light (about 580 nm). Hence the dichroism of the dichroic prism varies greatly for p-polarized light versus s-polarized light.
In the prior art color projection device shown in FIG. 6, the light flux that is projected on the screen is the light flux that has been modulated from s-polarized light to p-polarized light by the LCLVs 1R, 1G, and 1B. The dichroic prisms 17 and 18 separate color light according to the dichroism relative to s-polarized light. The dichroic prisms 17 and 18 combine the modulated color light according to the dichroism relative to p-polarized light.
Since the dichroisms for s-polarized light and p-polarized light vary as discussed above, a significant amount of light reflected by the LCLVs into the dichroic prisms 17, 18 to be combined and projected is lost (i.e., does not contribute to the projected image). Additionally, the difference in dichroism for the different polarized lights results in an image having poor color balance. Specifically, the amount of light actually projected may be determined by the product of the dichroism attained when the s-polarized light passes through the dichroic prism and the dichroism attained when the p-polarized light passes through the dichroic prism.
In the prior art projection device shown in FIG. 6, if dichroic mirrors are substituted for the dichroic prisms 17 and 18 and the dichroic mirrors are located obliquely with respect to an optical axis as shown, astigmatism occurs. The astigmatism deteriorates the resolution of a projected image.
Moreover, the prior-art projection device shown in FIG. 6 and described above projects an image having poor contrast. Specifically, whenever the LCLVs fail to modulate light according to the color signals, the reflected light flux emanating from the LCLVs is s-polarized. The s-polarized light is combined by the dichroic prisms and discarded by the polarizing beam splitter. Consequently, the contrast of the projected image on the screen is poor.
Particularly, in the FIG. -6 device, when linearly polarized light produced by the PBS 16 falls on a dichroic prism, the directions of s-polarized light and p-polarized light are determined by a vector n having a direction normal to the dichroic film of the dichroic prism and a vector T in the propagation direction of incident light. A vector S in the direction of s-polarization, is expressed as S=nxc3x97T.
Whenever a light flux is incident on a dichroic prism in such a manner that the linear polarization plane is not in the plane defined by a line normal to the dichroic film surface and the propagation direction of the incident light, the light flux is separated into s-polarized light and p-polarized light, and a phase difference occurs between the s-polarized light and p-polarized light. As a result of the change in the state of polarization of the light flux, the resultant light flux typically behaves as elliptically polarized light. Unwanted polarized light is, thus, superposed on light that is projected on the screen. This results in a deterioration of the blackness and contrast of a projected image on the screen.
In view of the foregoing short comings of the prior art, an object of the present invention to provide a color projection apparatus having a simply constructed polarizing optical system including a plurality of dichroic mirrors capable of projecting a high-contrast image without encountering a change in the state or phase of polarization of the light flux. Additionally, an object of the present invention is to provide a color projection apparatus that projects an ideal black image, excellent image color balance, and contrast (i.e., resolution).
To attain dichroism that varies insignificantly between p-polarized light and s-polarized light, it has been found that dichroic mirrors formed by coating a dichroic film on the surface of, for example, a glass plate, can be substituted for the dichroic prisms. The dichroism of a dichroic mirror can be illustrated in FIG. 8. The dichroic mirror is coated with a dichroic film identical to the dichroic film in the dichroic prism having the dichroism shown in FIG. 7.
The curve in FIG. 8 is based on the assumption that the s-polarized light and the p-polarized light fall on the dichroic mirror at an angle of incidence of about 45xc2x0. In FIG. 8, transmittance values are recorded on the Y axis and wavelength values on the X axis. The transmission/reflection boundary wavelength for the p-polarized light is approximately 500 nm and the transmission/reflection boundary wavelength of the s-polarized light is approximately 520 nm. The difference in transmission/reflection boundary wavelengths for the p-polarized light versus the s-polarized light is, thus considerably reduced compared to FIG. 7.
According to a first embodiment, a color-projection apparatus is provided comprising : (1) a polarizing beam splitter (PBS) for splitting a light flux emitted from a light source; (2) dichroic mirrors for separating a light flux polarized in a single direction, which light flux has been split by the PBS into blue (B), green (G), and red (R) color lights; and (3) reflection-type light valves for modulating the respective R, G, B color lights obtained by the respective dichroic mirrors.
Color lights emerging from the LCLVs are incident on the dichroic mirrors. The dichroic mirrors combine the color lights into a single light flux. The combined light flux is incident on the PBS and is projected by a projection lens onto a screen or other viewing surface. The PBS includes a polarization-splitting film operable to split incident light into s-polarized light and p-polarized light.
The dichroic mirrors are operable to separate a light flux into the primary color lights (R, G, B) and to combine the color lights R, G, B into a single light flux. The polarizing-splitting film of the PBS and the dichroic mirrors are disposed substantially parallel with one another so that the linear polarization plane is in the plane defined by lines normal to the dichroic mirror surfaces and the propagation direction of the incident light. Accordingly, the light flux is not separated into s-polarized light and p-polarized light, and a phase difference does not occur between s-polarized light and p-polarized light. Quarter-wave plates are disposed along the optical path between the corresponding dichroic mirror and the light valve of each respective primary color.
According to a second embodiment, a color projection apparatus is provided that differs from the first embodiment in that the elements of the second embodiment are arranged so that the PBS transmits a p-polarized light from an incident illumination-light flux to fall on a dichroic mirror while discarding the s-polarized light.
According to a third embodiment, a color-projection apparatus is provided that comprises: (1) a PBS for splitting an illumination-light flux emitted from a light source; (2) a dichroic mirror for separating the first polarized light into green light and light containing both blue light and red light; (3) a dichroic prism for separating the light, comprising both blue and red light, into blue light and red light; and (4) LCLVs for modulating the respective R, G, B color lights produced by the dichroic mirror and the dichroic prism. The dichroic mirror is operable to: (1) substantially transmit or reflect s-polarized light having wavelengths less than a first transmission/reflection boundary wavelength (a xe2x80x9cfirst xcexxe2x80x9d) that is near a boundary wavelength between green light and blue light (about 500 nm), (2) substantially transmit or reflect s-polarized light having wavelengths greater than a second transmission/reflection boundary wavelength (a xe2x80x9csecond xcexxe2x80x9d) that is near a boundary wavelength between green light and red light (about 590 nm), (3) substantially reflect or transmit s-polarized light having wavelengths within a wavelength range of from about the first xcex to about the second xcex, (4) substantially transmit or reflect p-polarized light having wavelengths less than a third transmission/reflection boundary wavelength (a xe2x80x9cthird xcexxe2x80x9d) that is near the boundary wavelength between the green light and blue light (about 500 nm), (5) substantially transmit or reflect p-polarized light having wavelengths greater than a fourth transmission/reflection boundary wavelength (a xe2x80x9cfourth xcexxe2x80x9d) that is near the boundary wavelength between the green light and red light (about 590 nm), and (6) reflect or transmit p-polarized light having wavelengths within a wavelength range of from about the third xcex to about the fourth xcex.
The dichroic prism in the third embodiment is operable to: (1) substantially transmit or reflect s-polarized light having wavelengths less than a fifth transmission/reflection boundary wavelength (a xe2x80x9cfifth xcexxe2x80x9d), (2) substantially reflect or transmit s-polarized light having wavelengths greater than the fifth xcex, (3) substantially transmit or reflect p-polarized light having wavelengths less than a sixth transmission/reflection boundary wavelength (a xe2x80x9csixth xcexxe2x80x9d), and (4) substantially reflect or transmit p-polarized light having wavelengths greater than the sixth xcex, wherein (5) the fifth xcex and the sixth xcex are within a wavelength range of from about the first xcex or third xcex to about the second xcex of the fourth xcex.
A projection apparatus according to the third embodiment provides the advantage that the dichroic mirror employed has a small difference in dichroism between s-polarized light and p-polarized light. Thus, only significant amounts of light are lost and more light contributes to the projected image. Additionally, the dichroic prism that exhibits a larger difference in dichroism between s-polarized light and p-polarized light provides the advantage of excellent image contrast or resolution. Consequently, light loss is suppressed, image color balance is improved, and deterioration of image resolution or contrast is minimized.
Additionally, according to the third embodiment, the projection apparatus includes both a dichroic mirror and a dichroic prism. Because a dichroic prism is not significantly affected by the external environment (unlike a dichroic mirror), the overall system is less susceptible to environmental influences compared to a system having two dichroic mirrors.
A color-projection apparatus according to a sixth embodiment of the present invention includes one or more quarter-wave plates. The quarter-wave plates are arranged between the dichroic mirror and the corresponding LCLV and/or between a dichroic prism and the corresponding LCLV. The quarter-wave plate(s) are, preferably, disposed substantially perpendicular to the optical axis. Axes of advancement or axes of retardation of the quarter-wave plates, are preferably, substantially contained on planes defined by lines normal to the dichroic film of the dichroic mirror and/or the dichroic prism and the optical axis. Whenever the quarter-wave plates are arranged as described, separated-then-combined color light directed to the PBS contains no significant amount of undesirable polarized light component. Accordingly, the blackness and contrast of the projected image are significantly improved.
The fourth embodiment preferably further includes a second trimming filter interposed between the dichroic prism and the corresponding LCLV for eliminating red light, separated by and emitted from the dichroic prism, having wavelengths near the wavelength band of green light (from about 500 nm to about 560 nm).
According to a fifth embodiment of the color projection apparatus of the present invention, a dichroic film of a PBS, a dichroic film of a dichroic mirror, and a polarizing-splitting film of a dichroic prism are disposed substantially parallel with one another. With such an arrangement, the directions of p-polarization and s-polarization defined by the dichroic films are substantially consistent with one another. Accordingly, light flux entering the PBS after contacting the various dichroic films includes only relatively small amounts of undesirable polarized light components. Deterioration of the blackness and contrast of the projected image is minimized.
A color-projection apparatus according to a sixth embodiment of the present invention includes one or more quarter-wave plates. The quarter-wave plates are arranged between the dichroic mirror and the corresponding LCLV and/or between a dichroic prism and the corresponding LCLV. The quarter-wave plate(s) are, preferably, disposed substantially perpendicular to the optical axis. Axes of advancement or axes of retardation of the quarter-wave plates are, preferably, substantially contained on plates defined by lines normal to the dichroic film of the dichroic mirror and/or the dichroic prism and the optical axis. When the quarter-wave plates are arranged as described, separated-then-combined color light directed to the PBS contains no significant amount of undesirable polarized light component. Accordingly, the blackness and contrast of the projected image is significantly improved.
The foregoing and other features and advantages of the present invention will be more readily apparent from the following detailed description which proceeds with reference to the accompanying drawings.